1. Field of the Invention
This invention relates to a microscope whose overall magnification is changeable, and in particular to a microscope in which the image magnification obtained by an objective lens can be changed in multiple stages to change the range or size of an object to be examined seen in the field of view of the microscope.
2. Related Background Art
As a microscope of this type, there is known one shown in FIG. 8 of the accompanying drawings wherein an infinity area is formed between first and second objective lenses and further the overall magnification is adjustable by the use of a zoom lens.
That is, in FIG. 8, a light flux obtained from a sample 2 placed on a stage 1 is converted into a parallel light flux through a first objective lens L.sub.01, whereafter it is condensed by a second objective lens L.sub.02 and forms an intermediate image forwardly of a zoom lens L.sub.z through the intermediary of a first rectangular prism 3 and a mirror M1.
The zoom lens L.sub.z is designed to re-image the intermediate image formed forwardly of the zoom lens L.sub.z on an intermediate image plane 5 through the intermediary of mirrors M2, M3 and M4 and a second rectangular prism 4.
Thus, the intermediate image plane 5 is formed above the second objective lens L.sub.02, and an intermediate image of a magnification M.sub.0 determined by the first objective lens L.sub.01, the second objective lens L.sub.02 and the zoom lens L.sub.z can be formed on the intermediate image plane 5, and the magnification can be varied continuously within a predetermined range.
Accordingly, when the sample 2 is to be observed through an eyepiece L.sub.E provided above the intermediate image plane 5, the sample 2 can be observed at an overall magnification M represented by the following equation: EQU M=M.sub.0 .multidot.M.sub.E (1)
where M is the magnification of the eyepiece L.sub.E.
Thus, by providing the zoom lens L.sub.z whose magnification can be adjusted in addition to the first objective lens L.sub.01 and the second objective lens L.sub.02, the magnification M.sub.0 of the intermediate image can be varied continuously and the overall magnification M can be adjusted to a desired magnification other than the magnification determined by a combination of the first and second objective lenses L.sub.01 and L.sub.02 and the eyepiece L.sub.E. In this case, the size and range of the image for the field of view of the microscope are determined by the magnification of the image by the first and second objective lenses L.sub.01 and L.sub.02 and the zoom lens L.sub.z.
However, if the zoom lens L.sub.z is thus added in addition to the first objective lens L.sub.01 and the second objective lens L.sub.02, the number of optical elements is increased by the addition of the zoom lens L.sub.z, and this has led to the problem that flare occurs or the transmittance is reduced to degrade the quality of the image.
Further, the fact that the prisms 3 and 4 and the mirrors M1-M4 are provided and the optical path from the first objective lens L.sub.01 to the intermediate image plane 5 is bent and the zoom lens L.sub.z is provided leads not only to the problem that the microscope body becomes bulky, but also to the problem that the construction of the entire microscope becomes complicated.
Incidentally, if the prisms 3 and 4 and the mirrors M1-M4 are eliminated and the optical path from the first objective lens L.sub.01 to the intermediate image plane 5 is made straight and the zoom lens L.sub.z is disposed on this optical path, the construction of the whole can be simplified correspondingly to the elimination of the prisms 3 and 4 and the mirrors M1-M4.
However, even if this is done, the zoom lens L.sub.z will become necessary, and this leads to the problem that the construction of the whole becomes correspondingly complicated and the quality of the image is degraded. Further, if the optical path is made straight and the zoom lens L.sub.z is disposed therein, the eyepiece L.sub.E will unavoidably have to be disposed at a high position distant from the sample 2, and this has led to the problem that the microscope becomes very inconvenient to use.